Although lithium ion batteries are the current mainstream of batteries for cellular phones, due to the ever-increasing trend toward smaller sizes, lighter weight and higher functionality of mobile terminal devices (such as for continuously viewing a digital television program, continuously playing music, browsing WEB sites, etc.), it is anticipated that the cycle of charging and discharging a battery will be accelerated, and as a result, the life of a battery will tend to be shortened relative to the period of use of a mobile terminal device.
Here, methods of detecting the life of a battery include a method in which the life of a battery is determined by way of a change in an impedance value of the battery (for example, see Patent Document 1), and a method in which a function regarding a charging/discharging curve is stored in advance, and the life of a battery is determined by calculating and weighting a deterioration value when charging and discharging (for example, see Patent Document 2).
Moreover, in general, the life of a battery is defined by each battery manufacturer, and refers to a state in which the capacitance has been decreased to about 50 to 60% of the initial capacitance due to, for example, about 500 to 600 times charging and discharging. In addition, beyond this state, the expected operation of the mobile terminal device cannot be guaranteed due to an increase of internal resistance, etc.
Therefore, it is necessary to accurately and clearly output the life of a battery to the user in order to guarantee expected operations of the mobile terminal device.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H05-74501    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2004-264303